Biomedical Engineering Reference
In-Depth Information
(thick tissue), optical instruments should operate in the near-infrared (NIR) spectral
window in which biological tissues exhibit relatively weak absorption. This spectral
range, also known as the “therapeutic” window (wavelengths between
2
[700 and
1,000] nm), allows in vivo imaging through several centimeters of tissue (up to
15
cm in breast tissue). Then the tissue properties of interest can be limited to three
main parameters: scattering, absorption, and fluorescence.
The first major phenomenon that rules light propagation is elastic scattering. On
a microscopic scale, the sharp transitions of the refraction index induce a change
in the direction of light propagation. The overall effect of these light redirections
occurring at different scales partially characterizes the propagating medium. Light
scattering is a wavelength-dependent phenomenon conveying significant informa-
tion about tissue structure. In particular, scattering depends on the sizes, indices
of refraction, and structures of the denser subcellular components. Experimental
evidence demonstrates that nuclear size [
1
], cell membranes [
2
], and mitochondria
[
3
] are the major contributors to scattering in vivo. The cell nuclei (5-10 m)
are appreciably larger than the probing optical wavelength (0:01-1m). They
predominantly scatter light in the forward direction, and there is also appreciable
scattering in the backward direction (Mie scattering). Mitochondria are oblong
scatterers with a 1-4
m length and a 0:2-0:5
m diameter and are responsible
for scattering at larger angles (Rayleigh scattering). Contrasts in the size, density,
distribution, and refraction index of these organelles generate optical signatures that
may be disease specific [
4
].
The second main phenomenon that rules light propagation is absorption.
Absorption corresponds to a loss on energy associated with molecular electronic
transitions. The absorbed energy is then transformed into heat through nonradiative
decay or, less frequently, light of lower energy (fluorescence). The main molecules
exhibiting strong absorption characteristics, or chromophores, are oxy- and
deoxyhemoglobin, melanin, water, lipids, porphyrins, NADH, flavins, and other
structural components. However, when imaging thick tissue, due to the large volume
probed by the photons, the predominant light-absorbing molecules can be further
reduced to hemoglobin, water, and lipid. Other molecules contribute negligibly
to the overall absorption due to their relatively smaller absorption cross section,
low concentration, and/or their confined biodistribution. The main chromophores
exhibit spectrally dependent absorbing properties that permit to distinguish between
them and quantify their concentration in vivo when optical data is collected
at multiple wavelengths. Especially, the ability to quantify independently oxy-
and deoxyhemoglobin is unique among clinical imaging modalities, making
optical techniques a desirable functional imaging modality in numerous clinical
scenarios.
Fluorescence is an absorption-mediated phenomenon. It is related to an electronic
transition from the excited state to the ground state of a molecule. In some cases, this
relaxation may generate a photon of lower energy. Such photon generation will be
specific to a molecular species and can be characterized by two intrinsic molecular
parameters: the quantum yield and the lifetime. The quantum yield is defined
by the ratio of the number of fluorescent photons emitted versus the number of
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